It is known that electromagnetic communication systems employ broad bandwidth techniques, such as the so-called frequency-agile or frequency-hopping systems in which both the transmitter and receiver rapidly and frequently change communication frequencies within a broad frequency spectrum in a manner known to both units. When operating with such systems, antennas having multiple matching and/or tuning circuits must be switched, whether manually or electronically, with the instantaneous frequency used for communications. As such, it is imperative to have a single antenna reasonably matched and tuned to all frequencies throughout the broad frequency spectrum of interest. Although the art discloses such broad band antennas, these antennas provide a somewhat limited frequency range.
As is well known in the art, a thin linear monopole antenna is normally used in a manner that requires its electrical length to be a quarter wavelength or 90 electrical degrees. These antennas require a ground plane, which is a large plane of sheet metal, such as a car or vehicle body made of metal, to provide the other half of the antenna. Therefore, the characteristics of the ground dependent “quarter wave” antenna are well known.
In order to enable a thin linear monopole antenna to be multi-band, the known art teaches placement of “traps,” which are parallel inductors and capacitors, at various places in series with thin linear radiators (conductors). Such a construction results in a monopole that can be used for several frequencies or very narrow bands of frequencies. Unfortunately, the useful bandwidths for this type of antenna are very narrow, usually on the order of KHz or 2-3 MHz. With this in mind, it would be presumed that additional traps in series at various points with the linear radiators should produce additional bandwidth. However, the number of traps is usually limited to 2 or 3. The reason for this is that adjustment of each trap to its specific frequency or operational bandwidth is interdependent on the adjustment of all the traps within the antenna.
The main purpose of utilizing a trap is to change the electrical length of the monopole radiator as the frequency of operation is changed. Moreover, at a specific trap's operational frequency or bandwidth, the current in the linear radiator physically above the trap in question, is reduced to or near zero so that the current distribution of the radiator physically below the trap in question is approximately that of a quarter-wave monopole radiator. In view of the interdependency of each trap in order to obtain a desired frequency bandwidth, there is currently not available in the art a linear monopole antenna with a bandwidth anywhere near 482 MHz. Nor is there available an antenna with such a wide bandwidth that also has a relatively low VSWR across the bandwidth.
One solution to the aforementioned problem is the use of inductive/resistive networks which create the electrical shortening process as the applied signal frequency is increased. Such an antenna is disclosed in U.S. Pat. No. 6,429,821 which is incorporated herein by reference. The problem is that these inductive/resistive networks as implemented are not perfect and can have serious parasitic effects from stray capacitance and self resonate effects of the inductive coils and especially the resistors which have to dissipate waste power via a heat sink. These resistors have inherent shunting capacitance because of the need to be coupled closely to a heat sink, that interferes with the ideal operation of the networks. The result is that these networks of the prior art allow “blow-bye” antenna currents that degrade the intended radiation pattern of the antenna.
One significant drawback to distributed inductive/resistive networks is their design inflexibility. The inductors in these networks can be changed as needed, but the resistors must be restricted to “first-order” networks because the power resistor needs a heat sink or thermal mass to dump waste heat. The ability to select the appropriate resistor which uses the preferred “thick film” technology is difficult because of the relatively expensive and slow manufacturing process of the resistors used in such networks. Indeed, higher-order networks require several power resistors to be arranged on a heat-sink in more complicated geometries which makes design and practical application of such networks even more difficult.
One solution to the blow-bye problem is to use of ferrite/powdered iron networks in the form of toroidal cores. It is known that toroidal ferrite cores can be used to minimize high frequency noise by absorbing the excess noise or energy ultimately to the conversion of heat. Further, the ability of the toroidal ferrite to absorb radio frequency can be carefully characterized by testing to select toroidal dimensions, core material type, and integrated heat transfer medium that best mimics a perfect inductor resistor networks in a parallel configuration. In theory, it appears to be easy to match the performance of a inductive/resistive based antenna with an antenna based on ferrite cores (also referred to as beads). However, it is harder to match a ferrite bead antenna with an inductive/resistive based antenna because in some cases the inductive/resistive networks must be arranged in more complex/exotic geometries or “high order” filter configurations to realize the same amount of antenna surface current control. Concerns also arise regarding the specific composition of the ferrite material. In any event, the ferrite beads or cores are selected to be electrically similar to the lumped inductive/resistive networks, and do not suffer from the above described parasitic effects. However, the use of such toroidal cores for high power antenna designs were thought to be impractical because of severe over-heating of the cores and as a result, fracturing of these cores and/or inadvertently ruining the magnetic properties by over heating them past their Curie temperature. The present invention incorporates an integrated heat dissipative system that allows the otherwise destructive heat build up to be safely dissipated away while minimizing the side effects of the prior art's parasitic capacitance caused by the use of power resistors and the necessary close-coupled heatsink.